refactor(engines): move plugin engines into an Engines/ subfolder

Plugin engines (Opus55, Qwen, Terra) each add two projects at the repo
root, and more are coming; at a dozen they would outnumber the core
projects. They are also a different kind of thing: out-of-solution,
runtime-loaded plugins. Grouping them under Engines/ keeps the root
readable.

Engines/Directory.Build.props now holds the shared TFM, nullable and
implicit-usings settings and the OpenNest.Engine reference, so a new
engine's csproj is nearly empty. The tests/ compile exclusion lives in
Directory.Build.targets because a removal in .props runs before the SDK
adds its default Compile glob and has no effect.

Build-Engines.ps1 replaces the per-README manual build-and-copy steps
for deploying engines into the benchmark's runtime Engines/ folder.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Build-Engines.ps1

@@ -0,0 +1,40 @@
+<#
+.SYNOPSIS
+    Builds every plugin engine under Engines/ and deploys it to the benchmark.
+
+.DESCRIPTION
+    OpenNest.Benchmark loads plugin engines from an Engines/ folder next to its own
+    build output. This builds the benchmark plus each Engines/OpenNest.Engine.*/ project
+    (test subprojects are skipped) and copies each engine DLL into that folder.
+
+.EXAMPLE
+    ./Engines/Build-Engines.ps1
+    ./Engines/Build-Engines.ps1 -Engines Opus55,Terra -Configuration Debug
+#>
+param(
+    [string]$Configuration = 'Release',
+    # Engine names without the OpenNest.Engine. prefix; default is all of them.
+    [string[]]$Engines
+)
+
+$ErrorActionPreference = 'Stop'
+$repoRoot = Split-Path $PSScriptRoot -Parent
+
+dotnet build (Join-Path $repoRoot 'OpenNest.Benchmark/OpenNest.Benchmark.csproj') -c $Configuration
+if ($LASTEXITCODE -ne 0) { throw 'OpenNest.Benchmark build failed.' }
+
+$deployDir = Join-Path $repoRoot "OpenNest.Benchmark/bin/$Configuration/net8.0/Engines"
+New-Item -ItemType Directory -Force $deployDir | Out-Null
+
+$projects = Get-ChildItem $PSScriptRoot -Directory -Filter 'OpenNest.Engine.*' |
+    Where-Object { -not $Engines -or $Engines -contains $_.Name.Substring('OpenNest.Engine.'.Length) }
+
+foreach ($dir in $projects) {
+    $csproj = Join-Path $dir.FullName "$($dir.Name).csproj"
+    dotnet build $csproj -c $Configuration
+    if ($LASTEXITCODE -ne 0) { throw "$($dir.Name) build failed." }
+
+    $dll = Join-Path $dir.FullName "bin/$Configuration/net8.0/$($dir.Name).dll"
+    Copy-Item $dll $deployDir -Force
+    Write-Host "Deployed $($dir.Name) -> $deployDir"
+}

Directory.Build.props

@@ -0,0 +1,15 @@
+<Project>
+  <!--
+    Shared settings for out-of-solution nesting engine plugins. Each engine lives in
+    Engines/OpenNest.Engine.<Name>/ with an optional tests/ subproject; both import this.
+    AssemblyName and RootNamespace default to the project file name.
+  -->
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+  <ItemGroup>
+    <ProjectReference Include="$(MSBuildThisFileDirectory)../OpenNest.Engine/OpenNest.Engine.csproj" />
+  </ItemGroup>
+</Project>

Directory.Build.targets

@@ -0,0 +1,10 @@
+<Project>
+  <!--
+    Each engine's tests/ subproject sits inside the engine folder. This must live in a
+    .targets file: removals in Directory.Build.props run before the SDK adds its default
+    Compile glob, so they would have no effect.
+  -->
+  <ItemGroup>
+    <Compile Remove="tests/**/*.cs" />
+  </ItemGroup>
+</Project>

OpenNest.Engine.Opus55/OpenNest.Engine.Opus55.csproj

@@ -1,14 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
-  <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
-    <RootNamespace>OpenNest.Engine.Opus55</RootNamespace>
-    <AssemblyName>OpenNest.Engine.Opus55</AssemblyName>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
-  </PropertyGroup>
+  <!-- Shared settings and the OpenNest.Engine reference come from Engines/Directory.Build.props. -->
   <ItemGroup>
-    <Compile Remove="tests/**/*.cs" />
     <InternalsVisibleTo Include="OpenNest.Engine.Opus55.Tests" />
-    <ProjectReference Include="../OpenNest.Engine/OpenNest.Engine.csproj" />
   </ItemGroup>
 </Project>

OpenNest.Engine.Opus55/README.md

@@ -61,8 +61,8 @@ Every placement decision (which part, which rotation, where, on which sheet) com
 ## Build / test
 
 ```bash
-dotnet build OpenNest.Engine.Opus55/OpenNest.Engine.Opus55.csproj -c Release
-dotnet test  OpenNest.Engine.Opus55/tests/OpenNest.Engine.Opus55.Tests.csproj
+dotnet build Engines/OpenNest.Engine.Opus55/OpenNest.Engine.Opus55.csproj -c Release
+dotnet test  Engines/OpenNest.Engine.Opus55/tests/OpenNest.Engine.Opus55.Tests.csproj
 ```
 
 This project is intentionally **outside** `OpenNest.sln`, the same pattern as the
@@ -73,10 +73,12 @@ This project is intentionally **outside** `OpenNest.sln`, the same pattern as th
 ```bash
 dotnet build OpenNest.Benchmark/OpenNest.Benchmark.csproj -c Release
 mkdir -p OpenNest.Benchmark/bin/Release/net8.0/Engines
-cp OpenNest.Engine.Opus55/bin/Release/net8.0/OpenNest.Engine.Opus55.dll OpenNest.Benchmark/bin/Release/net8.0/Engines/
+cp Engines/OpenNest.Engine.Opus55/bin/Release/net8.0/OpenNest.Engine.Opus55.dll OpenNest.Benchmark/bin/Release/net8.0/Engines/
 dotnet OpenNest.Benchmark/bin/Release/net8.0/OpenNest.Benchmark.dll <path-to-.nest-or-manifest-or-folder>
 ```
 
+Or build and deploy in one step with `./Engines/Build-Engines.ps1 -Engines Opus55`.
+
 The engine reports as `Opus55NestingEngine`.
 
 ## Known limitations

OpenNest.Engine.Opus55/tests/OpenNest.Engine.Opus55.Tests.csproj

@@ -1,8 +1,5 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
     <IsPackable>false</IsPackable>
     <IsTestProject>true</IsTestProject>
   </PropertyGroup>
@@ -14,8 +11,7 @@
   <ItemGroup>
     <Using Include="Xunit" />
     <ProjectReference Include="../OpenNest.Engine.Opus55.csproj" />
-    <ProjectReference Include="../../OpenNest.Engine/OpenNest.Engine.csproj" />
     <!-- The benchmark's NestValidator is the arbiter the engine is scored by. -->
-    <ProjectReference Include="../../OpenNest.Benchmark/OpenNest.Benchmark.csproj" />
+    <ProjectReference Include="../../../OpenNest.Benchmark/OpenNest.Benchmark.csproj" />
   </ItemGroup>
 </Project>

OpenNest.Engine.Qwen/OpenNest.Engine.Qwen.csproj

@@ -1,13 +1,3 @@
 <Project Sdk="Microsoft.NET.Sdk">
-  <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
-    <RootNamespace>OpenNest.Engine.Qwen</RootNamespace>
-    <AssemblyName>OpenNest.Engine.Qwen</AssemblyName>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
-  </PropertyGroup>
-  <ItemGroup>
-    <Compile Remove="tests/**/*.cs" />
-    <ProjectReference Include="../OpenNest.Engine/OpenNest.Engine.csproj" />
-  </ItemGroup>
+  <!-- Shared settings and the OpenNest.Engine reference come from Engines/Directory.Build.props. -->
 </Project>

OpenNest.Engine.Qwen/README.md

@@ -31,7 +31,7 @@ engines to start; it must not be the same algorithm re-derived through indirecti
 ## Build
 
 ```bash
-dotnet build OpenNest.Engine.Qwen/OpenNest.Engine.Qwen.csproj
+dotnet build Engines/OpenNest.Engine.Qwen/OpenNest.Engine.Qwen.csproj
 ```
 
 This project is intentionally **outside** `OpenNest.sln` (same pattern as the
@@ -44,14 +44,16 @@ as part of the main solution.
 own build output:
 
 ```bash
-dotnet build OpenNest.Engine.Qwen/OpenNest.Engine.Qwen.csproj -c Release
+dotnet build Engines/OpenNest.Engine.Qwen/OpenNest.Engine.Qwen.csproj -c Release
 dotnet build OpenNest.Benchmark/OpenNest.Benchmark.csproj -c Release
 
 mkdir -p OpenNest.Benchmark/bin/Release/net8.0/Engines
-cp OpenNest.Engine.Qwen/bin/Release/net8.0/OpenNest.Engine.Qwen.dll    OpenNest.Benchmark/bin/Release/net8.0/Engines/
+cp Engines/OpenNest.Engine.Qwen/bin/Release/net8.0/OpenNest.Engine.Qwen.dll    OpenNest.Benchmark/bin/Release/net8.0/Engines/
 
 dotnet OpenNest.Benchmark/bin/Release/net8.0/OpenNest.Benchmark.dll <path-to-.nest-or-folder>
 ```
 
+Or build and deploy in one step with `./Engines/Build-Engines.ps1 -Engines Qwen`.
+
 Your engine will show up in the report under its CLR type name (`QwenNestingEngine`),
 competing on equal footing against the built-in engines.

OpenNest.Engine.Qwen/tests/OpenNest.Engine.Qwen.Tests.csproj

@@ -1,8 +1,5 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
     <IsPackable>false</IsPackable>
     <IsTestProject>true</IsTestProject>
   </PropertyGroup>
@@ -14,6 +11,5 @@
   <ItemGroup>
     <Using Include="Xunit" />
     <ProjectReference Include="../OpenNest.Engine.Qwen.csproj" />
-    <ProjectReference Include="../../OpenNest.Engine/OpenNest.Engine.csproj" />
   </ItemGroup>
 </Project>

OpenNest.Engine.Terra/OpenNest.Engine.Terra.csproj

@@ -1,13 +1,3 @@
 <Project Sdk="Microsoft.NET.Sdk">
-  <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
-    <RootNamespace>OpenNest.Engine.Terra</RootNamespace>
-    <AssemblyName>OpenNest.Engine.Terra</AssemblyName>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
-  </PropertyGroup>
-  <ItemGroup>
-    <Compile Remove="tests/**/*.cs" />
-    <ProjectReference Include="../OpenNest.Engine/OpenNest.Engine.csproj" />
-  </ItemGroup>
+  <!-- Shared settings and the OpenNest.Engine reference come from Engines/Directory.Build.props. -->
 </Project>

OpenNest.Engine.Terra/README.md

@@ -31,7 +31,7 @@ engines to start; it must not be the same algorithm re-derived through indirecti
 ## Build
 
 ```bash
-dotnet build OpenNest.Engine.Terra/OpenNest.Engine.Terra.csproj
+dotnet build Engines/OpenNest.Engine.Terra/OpenNest.Engine.Terra.csproj
 ```
 
 This project is intentionally **outside** `OpenNest.sln` (same pattern as the
@@ -44,14 +44,16 @@ as part of the main solution.
 own build output:
 
 ```bash
-dotnet build OpenNest.Engine.Terra/OpenNest.Engine.Terra.csproj -c Release
+dotnet build Engines/OpenNest.Engine.Terra/OpenNest.Engine.Terra.csproj -c Release
 dotnet build OpenNest.Benchmark/OpenNest.Benchmark.csproj -c Release
 
 mkdir -p OpenNest.Benchmark/bin/Release/net8.0/Engines
-cp OpenNest.Engine.Terra/bin/Release/net8.0/OpenNest.Engine.Terra.dll    OpenNest.Benchmark/bin/Release/net8.0/Engines/
+cp Engines/OpenNest.Engine.Terra/bin/Release/net8.0/OpenNest.Engine.Terra.dll    OpenNest.Benchmark/bin/Release/net8.0/Engines/
 
 dotnet OpenNest.Benchmark/bin/Release/net8.0/OpenNest.Benchmark.dll <path-to-.nest-or-folder>
 ```
 
+Or build and deploy in one step with `./Engines/Build-Engines.ps1 -Engines Terra`.
+
 Your engine will show up in the report under its CLR type name (`TerraNestingEngine`),
 competing on equal footing against the built-in engines.

OpenNest.Engine.Terra/tests/OpenNest.Engine.Terra.Tests.csproj

@@ -1,8 +1,5 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
     <IsPackable>false</IsPackable>
     <IsTestProject>true</IsTestProject>
   </PropertyGroup>
@@ -14,6 +11,5 @@
   <ItemGroup>
     <Using Include="Xunit" />
     <ProjectReference Include="../OpenNest.Engine.Terra.csproj" />
-    <ProjectReference Include="../../OpenNest.Engine/OpenNest.Engine.csproj" />
   </ItemGroup>
 </Project>